Workaround for artifact during rendering of cubemaps whereby particles are not oriented consistently over different view directions. Algorithm is explained in this video: https://youtu.be/1hbcWhifuxE

AimParticlesAtCameraForCubemapRendering.cs

        private struct ParticleData
        {
            public ParticleSystem System;
            public ParticleSystem.Particle[] Particles;
            public Transform Camera;
            public ParticleSystemRenderSpace Alignment;
        }

        /// <summary>
        /// Collect particle systems which are in view and use billboard rendermode.
        /// </summary>
        private void PrepareParticleSystems(Transform _camera, CaptureFromCamera360 _capture)
        {
            m_lSystems.Clear();
            foreach(ParticleSystem ps in GameObject.FindObjectsOfType<ParticleSystem>())
            {
                ParticleSystemRenderer psr = ps.GetComponent<ParticleSystemRenderer>();
                if (ps.isPlaying && psr.isVisible && psr.renderMode == ParticleSystemRenderMode.Billboard)
                {
                    m_lSystems.Add(new ParticleData
                    {
                        System = ps,
                        Particles = new ParticleSystem.Particle[ps.main.maxParticles],
                        Camera = _camera,
                        Alignment = psr.alignment,
                    });
                    psr.alignment = ParticleSystemRenderSpace.World;
                }
            }
        }

        /// <summary>
        /// During video recording, manually aim all billboard particles towards cubemap camera to avoid edge artifacts.
        /// Algorithm is explained in this video: https://youtu.be/1hbcWhifuxE
        /// </summary>
        private void AimParticlesAtCamera()
        {
            foreach (ParticleData pd in m_lSystems)
            {
                int numParticlesAlive = pd.System.GetParticles(pd.Particles);

                Vector3 particleToCameraWorld = Vector3.zero;
                for (int i = 0; i < numParticlesAlive; i++)
                {
                    Vector3 particlePosWorld = pd.System.transform.TransformPoint(pd.Particles[i].position);
                    particleToCameraWorld = pd.Camera.position - particlePosWorld;
                    Debug.DrawLine(particlePosWorld, particlePosWorld + Vector3.Normalize(particleToCameraWorld), Color.red);
                    particleToCameraWorld.z *= -1.0f;
                    pd.Particles[i].rotation3D = Quaternion.LookRotation(particleToCameraWorld, Vector3.up).eulerAngles;
                }
                pd.System.SetParticles(pd.Particles, numParticlesAlive);
            }
        }
        private void RestoreParticleSystems(CaptureFromCamera360 _capture)
        {
            foreach (ParticleData pd in m_lSystems)
            {
                ParticleSystemRenderer psr = pd.System.GetComponent<ParticleSystemRenderer>();
                psr.alignment = pd.Alignment;
            }
        }